The most important factors in terms of efficiency when burning wood are:

• The air tightness of the stove
• The amount of radiating surface
• The type and dryness of wood
• The house insulation
• The way the stove is controlled  

An efficient stove will burn the wood completely but slowly rather than in a flash. Burning is controlled by regulating the flow of air into the firebox. 

Wood burns in three phases:

• Water evaporates
• Wood turns into charcoal and gases
• The charcoal burns  

Pre E.P.A. certified stoves let the gases go up the chimney unburned. These gases can represent 60% of the potential heat of the wood. To avoid this the stoves require oxygen mixed with the gases at a temperature of at least 350 to 600^oC. Some stoves have designs that brings secondary air to the combustion chamber to burn gases and particles that would normally go up the chimney unburned and by that they get a higher efficiency. A slow combustion heater is normally driven by primary air. By implementing secondary air you will burn gases and particles more efficiently at a lower temperature in the firebox. This is normally called Clean Burn technology and will increase the efficiency by almost 40%.  

To obtain high efficiency the wood has to be dry. Wet wood will in addition to generating less energy create creosote in the chimney when the combustion is incomplete.

Visit woodheat.org for more information about heating with wood.